9/15/2006

In today's issue of Science, a new paper by Canolty et al. demonstrates a different kind of frequency multiplexing than has been described previously. Using electrodes planted directly on the cortices of 5 epileptic patients, they were able to show "cross-frequency coupling" in which the amplitude and phase of neuronal oscillations between 80 and 150 Hz were strongly modulated by the amplitude and phase of theta rhythms.

Readers should note that brain oscillations between 80 and 150 Hz are called "high gamma," and have only recently been explored scientificially. These oscillations are too fast to be reliably detected from the scalp using traditional EEG techniques. Furthermore, because invasive EEG recording is dangerous, only patients with an existing need for these implants (such as epileptics) can be used as subjects.

Previous evidence has indicated that theta-band power may be related to executive function, insofar as theta has been shown to be sensitive to task demands. Canolty et al describe how across all their tasks and across all their subjects, over 85% of electrodes showed strong theta/high gamma coupling at significant levels (pThe authors also showed that the locations of electrodes showing theta & high gamma coupling were sensitive to change in task, such that the same task always evokes a similar "topography" of coupling, while different tasks evoke a variety of patterns. The authors conclude that this data further supports the idea that cross-frequency coupling (or, as John Lisman has termed it, multiplexed oscillations) are an organizing principle of neural computation.

Yeah, MEG might be an option - I don't know anything about the limitations of MEG though. I wish I had asked Erik about possible alternative approaches (the second author was visiting friends here a little while back).

The paper is interesting (as far as these difficult-to-interpret results go), but I don't think they showed that gamma is driven by theta, but only that they are coupled. If you look at Figure 1, for instance, it looks like the gamma power precedes the theta trough. It is impossible to tell, from this paper, what is driving what, or if they are both being driven by a third process. We can only say that they are correlated.

Hi Eric - I agree completely that they don't show gamma is driven by theta. The fact that they are correlated, with increases in that correlation at specific amplitudes and phases, is very interesting nonetheless.

Speculation time: I suspect that ultimately we would not find one oscillation really driving another. Unless I'm misunderstanding something, the usefulness of these synchronized multiplexed oscillations comes from the fact that they are independently generated and are responsible for somewhat independent processes (a crude example might be "search" via gamma, and "maintenance" via theta).